WARSAW (AFP) - President Barack Obama told Ukraine's president-elect Petro Poroshenko on Wednesday that the US was committed to his country in the long term, as pro-Russia separatists continued to battle Ukrainian troops in the country's east.

"The United States is absolutely committed to standing behind the Ukrainian people not just in the coming days, weeks, but in the coming years," Obama told reporters after talks with Poroshenko in his Warsaw hotel, adding that he had "been deeply impressed" by Poroshenko's vision.

Obama also announced the US would be increasing non lethal military aid to Ukraine, including night vision goggles.

Poroshenko, speaking English, thanked Obama for "continuous" support, which he said was "crucially important."

The US president met him in a show of US support for Ukraine's right to chart its own future, before an encounter later this week with Russia's Vladimir Putin.

Invited by Poland to mark a quarter century since its first free elections on June 4, 1989, Poroshenko was in Warsaw on his first trip abroad since being elected in May.

For Obama, Poland is the first leg of a European tour designed to assuage security concerns in eastern Europe following Russia's annexation of Crimea and what Washington says is an effort to destabilise Ukraine.

On Tuesday, he proposed a "European Reassurance Initiative" of up to US$1 billion (S$1.26 billion) to finance extra US troop and military deployments to "new allies" in Europe.

Nato defence ministers also agreed Tuesday a series of steps to bolster protection in eastern Europe after the Ukraine crisis, but insisted they were acting within the limits of a key post-Cold War treaty with Moscow.